The present invention relates to diene rubber compositions reinforced with an inorganic filler, which are intended for the manufacture of tires or of semi-finished products for tires, in particular for the treads of these tires.
In order to obtain the optimum reinforcement properties imparted by a filler in a tire tread and thus high wear resistance, it is known that this filler should generally be present in the elastomeric matrix in a final form which is both as finely divided as possible and distributed as homogeneously as possible. Now, such conditions can only be obtained insofar as this filler has a very good ability firstly to be incorporated into the matrix during mixing with the elastomer and to be disagglomerated, and secondly to be dispersed homogeneously in this matrix.
It is known that carbon black has such abilities, which is generally not true of inorganic fillers, in particular silicas, because, for reasons of mutual attraction, these inorganic filler particles have an irritating tendency to agglomerate together within the elastomeric matrix. The harmful consequence of these interactions is to limit the dispersion of the filler and hence the reinforcing properties to a substantially lower level than that which it would be theoretically possible to achieve if all the (inorganic filler/elastomer) bonds that could be created during the mixing operation were in fact obtained; these interactions furthermore tend to increase the consistency of the rubber compositions in the uncured state and therefore to make them more difficult to work (“processability”) than in the presence of carbon black.
Since fuel economies and the need to protect the environment have become priorities, it has proved necessary to produce tires having reduced rolling resistance, without adversely affecting their wear resistance.
This has been made possible in particular due to the use, in treads for these tires, of new rubber compositions reinforced with inorganic fillers, in particular specific silicas of the highly dispersible type, which are capable of rivaling a conventional tire-grade carbon black from the reinforcing point of view, while offering these compositions a lower hysteresis, which is synonymous with lower rolling resistance for the tires comprising them, and also improved grip on wet, snow-covered or icy ground.
Treads filled with such highly dispersible silicas (referred to as “HD” or “HDS” for “highly dispersible” or “highly dispersible silica”), usable in tires having low rolling resistance which are sometimes referred to as “Green Tires” because of the energy saving offered to the user (“Green Tire concept”), have been described in large numbers. Reference will be made in particular to patent applications EP 501 227, EP 692 492, EP 692 493, EP 735 088, EP 767 206, EP 786 493, EP 881 252, WO99/02590, WO99/02601, WO99/02602, WO99/06480, WO00/05300 and WO00/05301.
These documents of the prior art teach the use of silicas of HD type having a BET specific surface area of between 100 and 250 m2/g. In practice, one HD silica of high specific surface area forming a reference in the field of “Green Tires” is in particular the silica “Zeosil 1165 MP” (BET surface area equal to approximately 160 m2/g) sold by Rhodia. Use of this silica “Zeosil 1165 MP” makes it possible to obtain good compromises in terms of tire performance, in particular satisfactory wear resistance and rolling resistance.
The advantage of using a silica having a high specific surface area lies mainly in the possibility of increasing the number of bonds of the silica with the elastomer and therefore of increasing the level of reinforcement thereof. This is why it appears advantageous to use in rubber compositions for tire treads silicas having a high specific surface area, possibly greater than that conventionally used of the order of 160 m2/g, in order in particular to improve the wear resistance of these treads. Nevertheless, the dispersibility of the filler and the increase in its specific surface area are considered to be contradictory characteristics, because a large specific surface area assumes an increase in the interactions between objects of filler, and therefore poor dispersion thereof in the elastomeric matrix and difficult processing.